An array of image sensors or active pixel sensors detect the intensity of light received by the image sensors. The image sensors typically generate electronic signals that have amplitudes that are proportionate to the intensity of the light received by the image sensors. The image sensors can convert an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the image sensors. The electronic signals can be conditioned and sampled to allow image processing.
Integration of the image sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of image sensors along with analog and digital signal processing circuitry allow s electronic imaging systems to be low cost, compact and require low power consumption.
Historically, image sensors have predominantly been charged coupled devices (CCDs). CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Further, CCDs dissipate large amounts of power and suffer from image smearing problems. An alternative to CCD sensors are active pixel sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power.
FIG. 1 shows a cross-section of a prior art array of image sensors. This array of image sensors includes elevated photo diode sensors located over a substrate 100. An interconnection structure 110 electrically connects an N-layer 112 of the photo diode sensors to the substrate 100. An I-layer 114 is formed over the N-layer 112. A P-layer 116 is formed over the I-layer 114. The P-layer 116, the I-layer 114 and the N-layer 112 form the array of photo diode sensors. Conductive vias 120, 122, 124, 126 electrically connect anodes of a first photo diode sensor, a second photo diode sensor, a third photo diode sensor and a fourth photo diode sensor to the substrate 100. A transparent conductive layer 118 is located over the array of photo diode sensors, and electrically connects cathodes of the first photo diode sensor, the second photo diode sensor, the third photo diode sensor and the fourth photo diode sensor to the substrate 100.
The photo diode sensors conduct charge when the photo diode sensors receive light. The substrate 100 generally includes sense circuitry and signal processing circuitry. The sense circuitry senses how much charge the photo diode sensors have conducted. The amount of charge conducted represents the intensity of light received by the pixel sensors.
FIG. 2 is a schematic of a typical circuit included on the substrate 100 which is electrically connected to each photo diode sensor 136. The circuit includes a switch 132 which drives the cathode of each photo diode sensor 136 to an initial cathode voltage and charges a cathode capacitor 134. The switch 132 is subsequently opened and the cathode capacitor 134 discharges as the photo diode sensor 136 connected to the cathode capacitor 134 conducts charge. The rate in which the cathode capacitor 134 discharges is dependent upon the intensity of light received by the photo diode sensor 136 connected to the cathode capacitor 134. Therefore, the intensity of light received by the photo diode sensor 136 can be determined by sampling the voltage on the cathode capacitor 134 a period of time after the switch 132 has been opened.
The charge conducted by a reverse biased diode is generated in a space charge region and neutral region of the diode. Photo diode sensors are configured so that the charge conducted by a reverse biased photo diode sensor is generated in the space charged region. The magnitude of the charge conducted is directly dependent on the volume of the space charged region. In an array of photo diode sensors, the space charged region of each photo diode sensor extends beyond the physical boundaries of the photo diode sensor. The space charged region is defined by the electric field between the anode and the cathode of the photo diode sensor.
FIG. 3 is a plot which approximately represents the cathode voltage of each of the photo diode sensors 136 which are driven by an electronic circuit similar to the circuit shown in FIG. 2. The switch 132 of the circuit is opened at time 160. The cathode voltage of each photo diode sensor then decreases as the cathode capacitor 134 discharges as the photo diode sensor 136 connected to the cathode capacitor 134 conducts charge. The cathode voltage is sampled at time 162. The greater the intensity of received light, the lower the cathode voltage is at the time 162 that the cathode voltage is sampled. The cathode voltage stops decreasing at time 164 because the photo diode sensor has saturated.
If the intensity of the light received by the photo diode sensors 136 is too great, the photo diode sensors will saturate. That is, the cathode capacitor 134 will fully discharge before the cathode voltage is sampled. Saturation of the photo diode sensors can be avoided by either limiting the intensity of the received light or limiting the time the photo diode sensors are allowed to conduct charge due to exposure to light.
When the photo diode sensors saturate, the sensors no longer collect charge, and the electric fields across the photo diode sensors collapse. When the electric fields across the sensor collapse, charge is collected or trapped in defects within the I-layer of the sensors. The charge collected within the defects of the I-layer can cause the photo diode sensors to suffer from image lag.
Image lag generally occurs when photo diode sensors sense a series of progressive images. That is, the photo diode sensors detect several images, one after another. For example, an array of photo diode sensor can used to generate a video stream of images. Image lag occurs when an image being sensed affects an image sensed in the future. The future image is typically the next subsequent image.
Upon detection of a "next image" the charge collected within the defects will be released. The effect is that the inherent capacitance of the photo diode sensor discharges more quickly than the capacitance of the photo diode sensor would have discharged had charge not been collected within the defects. That is, the charge collected within the defects affects the sensing of the next subsequent image, causing image lag.
It is desirable to have an active pixel sensor array formed adjacent to a substrate which can detect a series of images without suffering from as much image lag as prior art active pixel sensor arrays. It is desirable that the active pixel sensor array include a structure which does not require extra formation processing steps.